Conventional aluminum flake is manufactured in a ball mill containing steel balls, aluminum metal, mineral spirits, and a fatty acid usually stearic or oleic. The steel balls flatten the aluminum and break it into flakes. When the ball milling is complete the slurry is passed through a mesh screen for particle sizing. Flakes too large to pass through the screen are returned to the ball mill for further processing. Flake of the proper size is passed through the screen and introduced to a filter press where excess solvent is separated from the flake. The filter cake is then let down with additional solvent. Such conventional aluminum flake typically has a particle size from about 2 to about 200 microns and a particle thickness from about 0.1 to about 2.0 microns. These flakes are characterized by high diffuse reflectance, low specular reflectance, rough irregular flake micro surface, and a relatively low aspect ratio.
Another process for making metal flakes is a process of Avery Dennison Corporation for making flakes sold under the designation Metalure. In this process both sides of a polyester carrier are gravure coated with a solvent-based resin solution. The dried coated web is then transported to a metallizing facility where both sides of the coated sheet are metallized by a thin film of vapor deposited aluminum. The sheet with the thin metal film is then returned to the coating facility where both sides of the aluminum are coated with a second film of the solvent-based resin solution. The dried coated/metal sheet is then transported again to the metallizing facility to apply a second film of vapor deposited aluminum to both sides of the sheet. The resulting multi-layer sheet is then transported for further processing to a facility where the coatings are stripped from the carrier in a solvent such as acetone. The stripping operation breaks the continuous layer into particles contained in a slurry. The solvent dissolves the polymer out from between the metal layers in the slurry. The slurry is then subjected to sonic treatment and centrifuging to remove the solvent and the dissolved coating, leaving a cake of concentrated aluminum flakes approximately 65% solids. The cake is then let down in a suitable vehicle and further sized by homogenizing into flakes of controlled size for use in inks, paints, and coatings.
Metal flakes produced by this process for use in printable applications such as inks are characterized by a particle size from about 4 to 12 microns and a thickness from about 150 to about 250 angstroms. Coatings made from these flakes have a high specular reflectance and a low diffuse reflectance. The flakes have a smooth mirror-like surface and a high aspect ratio. The coatings also have a high level of coverage per pound of flake applied when compared with metal flakes produced by other processes.
Flakes also are produced in a polymer/metal vacuum deposition process in which thin layers of vapor deposited aluminum are formed on a thin plastic carrier sheet such as polyester or polypropylene, with intervening layers of cross-linked polymers between the vapor deposited aluminum layers. The cross-linked polymer layers are typically a polymerized acrylate deposited in the form of a vaporized acrylate monomer. The multi-layer sheet material is ground into multi-layer flakes useful for their optical properties. Coatings produced from such multi-layer flakes tend to have a high diffuse reflectance and a low specular reflectance. The flakes have a low aspect ratio and undesired low opacity when made into an ink.
One objective of the present invention is to reduce the number of manufacturing steps and the resulting cost of making highly reflective metal flakes, although the process also reduces the coast of making other flake-like materials described below.
In addition to metal flakes, there are many industrial uses of glass (SiO2) flakes. Conventional glass flakes generally have a thickness range of about one to six microns and a diameter from about 30 to about 100 microns. These glass flakes can be used for additions to polymers and coatings to improve various functional properties. These include addition of glass flakes as additives to produce thinner, smoother coatings, for example. One objective of this invention is to produce very thin, flat, smooth flakes, such as metal or glass flakes, for example, for use of their various functional properties in polymers, coatings and films.